Makaira:
Congratulations on the purchase.
In response to your question, all factory 204 Ruger chambered rifles have 1 in 12" twist barrels.
Some folks have found that the 40 grain Hornady VMax may not shoot as well as the 32 grain VMax in their factory barrels. Most of those folks have found, however, that the 39 grain Sierra shoots great in the same barrel they get so-so accuracy with using the 40 grain bullet. If you hand load, it should not be a problem to use the heavier 39 grain Sierra bullets if the 40 VMax doesn't satisfy you.
I have owned 2 Ruger stainless Ultralites with the 20" barrel (foolishly sold the first and bought another) and a Remington 700 with a 24" barrel. All three shot the 40 grain Vmax bullets very well in limited use. I don't shoot a lot of factory bullets however, because I hand swage my own 20 caliber bullets.
I also own a couple of Tac 20 rifles with custom barrels - both with 12" twists and they both shoot the 40 grain Vmax accurately, again with limited use.
Hopefuly your rifle will shoot the 40's and if you hand load, I'm fairly certain the 39's will shoot great for you if need be. Lots of folks have reported excellent results with the Sierra bullets, both in accuracy and in terminal performance.
Good luck when you get your rifle. - BCB